HDU 1856 More is better（求最大秩的集合 ）

题目地址：点击打开链接

题意：给一群人挑出来人数最多的一群

思路：求最大秩的集合

AC代码：

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <algorithm>
#include <queue>
#include <stack>
#include <map>
#include <cstring>
#include <climits>
#include <cmath>

using namespace std;

const int maxn = 10000010;
int pre[maxn],num[maxn];


int find(int x)
{
	int r = x,i = x,j;
	while(pre[r] != r)
	{
		r = pre[r];
	}
	while(pre[i] != r)//路径压缩
	{
		j = pre[i];
		pre[i] = r;
		i = j;
	}
	return r;
}

/*int find(int x)
{
    if(pre[x] != x)
        return find(pre[x]);
    return x;
}*/
//没路径压缩会超时

/*if(find(x) != find(y))
{
    pre[find(y)] = find(x);
    num[find(x)] += num[find(y)];
}*/
void join(int x,int y)
{
    //上面一部分是错的，没看出来原因，这段代码效率也不高，find函数执行了6次，下面只执行了2次
    int p = find(x);
	int q = find(y);
	if(p != q)
    {
        pre[p] = q;
        num[q] += num[p];
    }
}

int main()
{
	int n,max1,i,x,y;
	while(scanf("%d",&n) != EOF)
	{
	    if(n == 0)
	    {
            printf("1\n");
            continue;
	    }
	    max1 = 0;
		for(i=1; i<=maxn; i++)
		{
			pre[i] = i;
			num[i] = 1;
		}
		for(i=0; i<n; i++)
		{
			scanf("%d%d",&x,&y);
			if(max1 < x)
                max1 = x;
            else if(max1 < y)
                max1 = y;
			join(x,y);
		}
		int max2 = 0;
		for(i=1; i<=max1; i++)
        {
            if(num[i] > max2)
                max2 = num[i];
        }
        //加个条件判断pre[i] == i也行
		printf("%d\n",max2);
	}
	return 0;
}